Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 242002
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1002 AM HST Sat Feb 24 2018

.SYNOPSIS...
Moist and somewhat unstable conditions will persist through the
weekend. East to southeast winds will focus rainfall over windward
and southeast slopes. Leeward areas will remain mostly dry with
only a chance of showers. The greatest threat for heavy showers
and thunderstorms will remain over the Big Island through tonight,
then shift to Kauai and Oahu Sunday. Drier conditions will push
over the state early next week as strong easterly winds develop
across the state.

&&

.DISCUSSION...
East to southeast winds are pushing showers and clouds into
windward areas as well as southeast facing slopes. Moderate rains
have continued through the night on the Big Island from the Hilo
area through the Puna and Kau Districts. Rain gages show moderate
24-hour rain totals from 1/2 to 3 inches. Windward areas of Maui
and Oahu are also seeing light to moderate rainfall this morning.

The cause of our current weather pattern is two-fold. One, a
strong mid to upper low is located NW of Kauai, and, two, a deep
high pressure is centered far northeast of the state. The upper
low is also supporting a surface trough west of the state. The
trough combined with the deep high pressure is enhancing ESE
winds across the state, and has brought strong and gusty winds to
exposed areas on the Big Island where a wind advisory remains in
effect. In addition to usual places affected by easterly winds
such as Kau and the Humuula Saddle, portions of the Hamakua Coast
will also experience localized gusty winds.

Models continue to show deep moisture east of the Big Island being
pushed ashore today with some tapering tonight and Sunday before
drier air moves in Monday. The upper low will continue the slight
chance of thunderstorms for the state through tonight and then
continue for only the western islands Sunday. The chance of rain
increases for Kauai and Oahu Sunday as a short wave trough swings
around the upper low bringing a shot of instability. Presently,
models are depicting a band of moderate to heavy showers moving
across Kauai and Oahu Sunday afternoon and evening before quickly
departing to the north. A flash flood watch may be issued for
Kauai and Oahu with the afternoon forecast package.

As the short wave lifts north, the upper low will move slowly
west Sunday night, allowing for some midlevel warming and a more
stable atmosphere to settle over the state Monday and into next
week. The deep high pressure will push closer to the state
bringing strong easterly trade winds with windward showers that
will occasionally blow over to leeward locations.

&&

.AVIATION...
Strong high pressure far NE of the area and a surface trough west
of the state will keep gusty ESE to SE winds over the islands
today. The strengthening winds speeds may require the issuance of
AIRMET TANGO for low level turbulence later today. This will
continue to be monitored. The more S terminals on the smaller
islands should see a bit weaker winds, due to the blocking from
the Big Island.

The SE winds will continue to advect moisture over the state.
Locally heavy showers, especially along the windward and SE facing
areas, can be expected today. Prolonged period of MVFR are
likely, with periods of embedded IFR. AIRMET SIERRA for mountain
obscuration is posted across those area. The PHTO terminal will
likely see prevailing MVFR through the day, with periods of IFR.
The other main terminals should stay predominately VFR, with
periods of MVFR possible. The slight threat of thunderstorms
still remains in the forecast for the entire state, with more
favorable areas being the Big Island and Maui slopes this
afternoon.

This wet, unstable SE pattern can be expected tomorrow.

&&

.MARINE...
An unstable airmass will continue the possibility for isolated
thunderstorms over all coastal waters today, before shifting to
the western half of the state Sunday.

A Small Craft Advisory (SCA) remains posted for most waters,
particularly on the east sides of the islands due to the strong
winds and elevated seas. See the Marine Weather Message (product
MWWHFO) on our website for the affected areas. Strong high
pressure far to the north- northeast will maintain breezy east to
southeast wind pattern through the remainder of the weekend, with
winds expected to strengthen further during the first half next
week. The SCA may need to be expanded next week as the winds
increase with possible gale force winds.

A long and rather broad fetch of easterly winds on the south side
of the subtropical high will continue to produce advisory level
surf along east facing shores into early next week. The latest
model guidance shows surf could reach warning levels as early as
Monday night or Tuesday. This combined with high astronomical
tides could cause some coastal inundation issues along some low
lying areas.

The current small northwest swell will continue to lower
gradually today. Otherwise surf along north and west facing
shores will be well below normal for this time of year during the
Sunday through Thursday time period. A series of small, mainly
background south swells can be expected through the weekend and on
into the middle of next week.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Monday for Kauai Windward-Oahu
Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

Wind Advisory until 6 PM HST this evening for Kona-South Big
Island-Big Island North and East-Kohala-Big Island Interior.

Small Craft Advisory until 6 AM HST Monday for Kauai Northwest
Waters-Kauai Windward Waters-Oahu Windward Waters-Kaiwi Channel-
Maui County Windward Waters-Alenuihaha Channel-Big Island
Windward Waters-Big Island Leeward Waters-Big Island Southeast
Waters.

&&

$$

Morrison...Discussion
Eaton...Aviation
Foster...Marine



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